Technical Field
The disclosure relates to an RF tag circuit that is connected to an antenna and a load.
Description of Related Art
In recent years, the development of RF tags has been remarkable, and use of RF tags for driving loads such as a sensor, an LED, an IC, a microcomputer, a communication circuit, and an RF tag circuit as examples using power supplied through received radio waves as an energy source has increased in addition to their original use of object identification. In some cases, the electric power received by RF tags is further increased such that the electric power is stably supplied for operations of such loads. In such an RF tag circuit, there are cases in which a power supply capability for a load decreases due to impedance mismatch between an antenna and an RF tag circuit. The impedance mismatch may occur due to a change in the impedance of the antenna, for example, caused by the attachment or approach of a metal piece, or dielectrics such as water or oil to the antenna. For this reason, technologies for impedance matching between an antenna and an RF tag circuit have been proposed (for example, see Japanese Patent Laid-open No. 7-111470).
There are many cases in which energy supplied through radio waves is insufficient for simultaneously performing driving of loads and other processes. For this reason, in many cases, in an RF tag circuit, a load is activated and driven at a predetermined cycle, and other processes are executed when the load is not driven.
In adjustment of the impedance, searching for an impedance value is performed such that the electric power generated by the RF tag circuit increases while the impedance of the RF tag circuit is changed. When the load is not driven, the voltage is saturated (for example, a state in which the voltage reaches a limit value according to a limiter). Accordingly, although the adjustment of the impedance is performed when the load is not driven, an increase in electric power is not detected, and it is difficult to appropriately adjust the impedance. For this reason, it is desirable to adjust the impedance while the load is driven.
In the search for an impedance value, it is desirable to adjust the impedance in a state in which the electric power generated by the RF tag circuit is stable. For this reason, a load driving period is determined such that a period until the electric power generated by the RF tag circuit is stabilized is secured. As a result, even when a load that can be driven in a time shorter than a period until the electric power generated by the RF tag circuit is stabilized is employed, it is difficult to shorten the load driving period.